According to the state of the art, automatic transmissions, in particular for motor vehicles, include planetary gear sets that are shifted by means of friction elements or shifting elements and are typically connected to a start-up element that is subject to a slip effect and is alternatively provided with a lock-up clutch, such as a hydrodynamic torque converter or a fluid coupling.
In general, automatically shiftable vehicle transmissions of planetary design are subject to continuous development and improvement. Thus, such transmissions should require low construction costs, in particular a small number of shifting elements, and, during sequential shifting operation, should avoid double gearshifts, i.e. the switching on and off of two shifting elements, such that, during gearshifts in defined gear groups, only one shifting element is changed.
A multi-speed transmission in planetary design is known from DE 10 2008 000 428 A1 of the applicant; this features an input and an output, which are arranged in a housing. With the known transmission, at least four planetary gear sets, hereinafter referred to as the first, second, third and fourth planetary gear sets, at least eight rotatable shafts—hereinafter referred to as the input shaft, output shaft, third, fourth, fifth, sixth, seventh and eighth shafts—along with at least six shifting elements, comprising brakes and couplings, are provided; their selective intervening brings about different transmission ratio relationships between the input and the output, such that, preferably, nine forward gears and one reverse gear are able to be realized.
Thereby, the first and the second planetary gear sets, which are preferably formed as negative planetary gear sets, thus with a negative stationary transmission ratio, form a shiftable upstream gear set, whereas the third and the fourth planetary gear sets form a main gear set.
According to the state of the art, the shifting elements of the multi-stage transmissions designed in such a manner, which are typically designed as multi-disk couplings or brakes, are hydraulically actuatable, which leads to high hydraulic losses in a disadvantageous manner. In order to avoid these actuating losses, the use of alternatively actuatable shifting elements, for example electromechanically actuatable shifting elements, would be particularly advantageous.
In order to enable the use of actuatable shifting elements in line with demand, the shifting elements, in particular the couplings, must be easily accessible from the outside.